In general, inkjet printing machines or printers include at least one printhead that ejects drops or jets of liquid ink onto a recording or image forming surface. An aqueous inkjet printer employs water-based or solvent-based inks in which pigments or other colorants are suspended or in solution. Once the aqueous ink is ejected onto an image receiving surface by a printhead, the water or solvent is evaporated to stabilize the ink image on the image receiving surface. When aqueous ink is ejected directly onto media, the aqueous ink tends to soak into the media when it is porous, such as paper, and change the size and location of the drop of ink from its intended position. To address this issue, a printer that ejects ink onto an intermediate surface has been developed. These printers are referred to as indirect printers in this document. One such printer ejects ink onto a rotating intermediate imaging surface, which is usually in the form of a rotating drum or endless belt. The ink is dried or partially dried on the member and then transferred to media. Such a printer avoids the changes in media properties that occur in response to media contact with the water or solvents in aqueous ink. Indirect printers also reduce the effect of variations in other media properties that arise from the use of widely disparate types of paper and films used to hold the final ink images.
In these indirect printers, the intermediate imaging surface has two competing requirements. The ink should adhere strongly to the location to which it was directed, yet be able to transfer from the intermediate imaging surface member to the media after it is dried. These goals can be achieved by applying a coating material to the blanket. Coating materials have a variety of purposes that include wetting the intermediate imaging surface, inducing solids to precipitate out of the liquid ink, providing a solid matrix for the colorant in the ink, and/or aiding in the release of the printed image from the intermediate imaging surface. Because the intermediate imaging surfaces are likely to be surfaces with low surface energy, reliable coating is a challenge. If the coating is too thin, it may fail to form a layer adequate to support an ink image. If the coating is too thick, a disproportionate amount of the coating may be transferred to media with the final image. Image defects arising from either phenomenon may significantly degrade final image quality.
Parameters other than coating thicknesses also affect image quality in an aqueous indirect inkjet printer. These parameters include coalescence of the ejected ink drops, spread of the ink drops, and inter-color bleed of adjacent ink drops in the process and cross process directions. These issues are not encountered or are not as severe in printing with other inks, such as solid or phase change inks, which become solid upon contact with the media. Also, the ink image changes as the ink dries. Consequently, evaluation of ink image status for high efficiency transfer of an ink image and coherence of the ink image for transfer is important and varies depending upon the position of the ink image in the print cycle. Moreover, after the ink image is transferred, the efficiency of the transfer and any subsequent cleaning of the intermediate imaging surface require evaluation as well. Analyzing the transfer of the ink image to the media and measuring the overall quality of the ink image on the media can also be important. Structuring printers and configuring the components in an aqueous printer to evaluate and adjust these various parameters at appropriate places in a printer remains a significant goal for making aqueous printers that reliably produce images on media with acceptable quality.